Overview
In the printing, converting, and coating industries ink and/or coating material is applied to a continuous web substrate either as an overall application or as a registered pattern. Variations within the materials as well as in the process itself can cause defects which can be minor and objectionable from both an aesthetic and functional point of view such as when they occur in printed images.
More serious are defects which occur in coatings that perform an important mechanical function. Two examples of coating applications in this category are:
1. Pattern coatings of cold seal (package sealants) used to provide and retain sterilization of the contents of medical instruments and supplies. Defects in the cold seal of the packaging render the contents non-sterile with serious potential for medical liability. PA0 2. The manufacture of convertible tops with continuous overall adhesive coating. Defective or inadequate coating in the overall adhesive which binds the inside insulation to the extruded vinyl outer layer results in premature delamination with high corresponding warrantee maintenance costs. Defects in the overall adhesive coating can result in premature delamination of the outer extruded vinyl material which is exposed to the elements and which is bonded to the substrate which would be inside of the car and not subject to the elements. PA0 1. A cylinder (136) contains a large number of recessed engraved cells where ever the coating is to be applied to the web (146). PA0 2. The engraved cylinder rotates in a bath of coating material contained in the coating tank (138). PA0 3. As the engraved cylinder rotates the cells fill with coating material. Doctor blade (137) wipes off all coating that is not trapped in the engraved cells with the result that coating appears only in those areas with engraved cells. PA0 4. Impression roller (135) exerts a large force pressing the web substrate (146) against engraved cylinder (136) whereupon the coating material is withdrawn from the cells and deposited on the web substrate in the same pattern as the etched cells. PA0 1. Voids that occur in the coated areas. Some of the cells in the coating areas do not transfer their coating material leaving tiny voids in areas that should contain solid homogeneous coating. PA0 2. Too little or too much coating material being applied. The coating solvent-solid mixture contains either too much or too little solid on a volume basis. PA0 1. The inspection procedure varies from operator to operator with large variations in quality due to variations in operator eyesight and observation diligence. PA0 2. The scope of inspection by a given operator is somewhat limited, and it is in general impossible to perform a full material defect detection at high material thruput rates. PA0 3. Long intervals with no inspection may occur resulting in poor quality and undetected waste as the operator is performing other manufacturing tasks. PA0 4. The manual visual method does not lend itself to the detection and correction of potential defects and lacks the capability of preventing material waste from occurring. Usually by the time defects are detected they are of sufficient magnitude that they cause manufacturing waste and must be removed. PA0 5. The manual visual procedure does not provide the capability of monitoring and recording quantitative product quality. PA0 1. Provide for an automatic inspection process with precise predetermined automatic defect detection to produce the same consistent product quality for all operators; PA0 2. Provide for a dedicated system that performs continuous inspection; PA0 3. Permit complete surface inspection of web coatings and the like in some circumstances; PA0 4. Provide for the detection of any size potential defect by using the magnification capabilities of a zoom lens; PA0 5. Permit corrective action in a timely fashion to prevent defective material production from occurring; PA0 6. Permit the quantification of defect size allowing the recording and storage of quality information and complete images that can be used for a number of purposes such as improving the process, verification of quality, and archiving material images for any reason where later recall may be beneficial; PA0 7. Provide for remote location of inspection cameras where an operator may not be able to inspect the coating material; PA0 8. Provide for inspection of both tinted and clear coating materials; PA0 9. Permit the coating thickness to be estimated in some circumstances; PA0 10. Permit high speed inspections of web coatings and the like; PA0 11. Permit a manufacturing feedback control system to be implemented using the defect size and quantity information automatically gathered by the defect detection system; PA0 12. Permit an automatic void detection system to provide for manufacturing feedback process control of impression pressure, solvent-to-solid mixture, and other process variables which may be adjusted by a typical manual printing operator; PA0 13. Permit feedback indicia for process changes which may compensate for wear of the engraved cylinder reducing cell volume, doctor blade wear, changes in the substrate, and the like.
While the herein described applications are shown for roll to roll processing, the same technique can be used on applications where the material is processed in sheet form.
Coating Processes
Rotogravure and flexography are the two most common coating processes that are capable of both pattern or registered coatings and are the focus of many of the exemplary embodiments of the present invention. These coating processes react similarly in regard to potential defects in coating applications. Thus, for simplicity purposes the rotogravure process will be used throughout this disclosure. A
To understand how variations occur in coatings, it is first necessary to understand how coatings are applied to webs in the rotogravure coating process. A rotogravure coating unit is shown as (107) in FIG. 1 and explained as follows:
The coating material itself is a solid which is dissolved in a suitable solvent and mixed in proportions to obtain a viscous fluid that will both fill the cells as the cylinder rotates and carry and deposit enough solid for the coating application. After the mixture is deposited on the substrate, the solvent evaporates leaving only the solid coating on the web. Generally heat is added after the mixture is deposited to increase the rate of evaporation and the speed of the process.
Types of Potential Coating Defects
For rotogravure coating applications the engraved cell dimensions are generally the same for all areas where the coating is applied. When applied correctly all coated areas are generally homogeneous or even textured. The two most common potential defects which if not corrected will cause waste are:
Causes of Potential Coating Defects
Each coating application is dependent upon the density and physical dimensions of the engraved cells and the correct ratio of solvent-to-solid material. Maintaining the ideal ratio of solvent-to-solid mixture is desired if optimum coating consistency is to achieved.
However, large variations in the solvent-solid mixture are common due to the difficulty of determining the correct amount of solvent or solid needed to replenish the original supply as it is being used. Solvent is also lost due to normal evaporation especially during long downtimes of the coating machines. Attempts to control the mixture have included the measurement and control of viscosity of the mixture, the use of large enclosed coating tanks, temperature control and others.
There are a number of other factors which cause the same type of defects as variations in solvent-to-solid ratio. As illustrated in FIG. 1, these include variations in pressure of the impression roller (135) against the engraved coating cylinder (136), wear of the engraved cylinder reducing cell volume, doctor blade wear and adjustment and absorbency of the substrate.